‘Robot probes – flashing through nearby star systems and radioing back their observations during a few hectic hours of transit … Barring accidents, they would continue speeding through the galaxy forever …’
Clarke, The Songs of Distant Earth, 1986
As Project Icarus enters its next stage we will, slowly but surely, move towards a precise quantitative design for our hypothetical interstellar probe.
But some interstellar craft in science fiction have featured remarkably detailed and plausible designs – not surprisingly, as many of them derived from work by, or were even written by, experts in the field. What can we learn from these fictional precedents?
In what follows I’m going to focus on ships of the Icarus/Daedalus class, very loosely speaking – that is, no much-slower-than-light multi-generation worldships, as featured from Heinlein’s Universe  to Greg Bear’s recent, and excellent, Hull Zero Three , and no faster-than-light behemoths like Star Trek’s Enterprise, probably the most detailed fictional starship of all, or the Alcubierre-drive starship of my own Ark  (2009). As this is a fairly informal blog I haven’t overloaded the article with specific technical references, but I’ve included pointers to sources where appropriate, mostly via Matloff’s recent survey .
Paul Gilster’s excellent Centauri Dreams  is, among other things, a good source of information on starships in sf. The earliest interstellar journey Paul identifies (p31) is by a French writer called C.L. Defontenay, dating from 1854; the ship was driven by a kind of antigravity. In sf’s pulp era of the early twentieth century, various fanciful space drives featured in the fiction of writers like EE Doc Smith, whose pioneering space opera Skylark of Space (1928)  appears to have featured a kind of inertial drive.
What was the first credible starship design in fiction? The earliest identified by Gilster ( p102) is a kind of solar sail, in a book called The Extraordinary Adventures of a Russian Scientist by the French writers Georges le Faure and Henri de Graffigny, published in 1889-91. It was after the quest for scientific rigour in sf pioneered by the great Astounding Science Fiction editor John W. Campbell from the late 1930s that starships of a more or less realistic design began to appear regularly in the genre.
Maybe it’s no surprise that the first realistic fictional starship was a lightsail. As far as I’m aware this is the only candidate interstellar propulsion technology whose underlying principles were thoroughly defined and indeed demonstrated by the end of the nineteenth century ( p103). Paul Gilster highlights an early and memorable depiction of a solar sail ship in Cordwainer Smith’s 1960 story ‘The Lady Who Sailed the Soul’ . But it was probably Clarke’s 1964 story ‘The Wind from the Sun’  that cemented the idea in the public consciousness.
Rigorous developments of the technology as a means of interstellar travel began with the work of Robert L. Forward. An early use of Forward’s ideas was by Niven and Pournelle in their 1974 novel The Mote in God’s Eye . Aliens from a star known as the Mote launch a 450,000-ton craft across 35ly (light years) at 7% of c (lightspeed) to the human-occupied ‘New Caledonia’ system. The ‘Moties’ use a laser-pushed lightsail; the sail is thousands of kilometres across and the laser blasts for 45 years. On its approach the ship itself is visible; it decelerates using the light of New Cal’s sun, which is reflected back blue-shifted (details  chapters 4 and 6). Forward however felt the authors glossed over the difficulties of deceleration: ‘I had warned them that the light from the Sun was not strong enough to stop the sail, but, being science fiction writers … they ignored my advice and pretended it would work’ ( p127).
Forward first presented his own deceleration design in the pages of a novel, Rocheworld , before writing it up for technical publication. In the 2020s, the twenty-strong crew of the Prometheus, the first manned starship, are to be propelled to Barnard’s Star by ‘a solar-system-wide machine that would toss them to the stars on a beam of light’ (p42). Forward goes into great technical detail, including graphs and diagrams.
The ship will make a forty-year, one-way journey across six light years to the target star. The payload massing ~3500t is to be dragged by a sail 1000km wide; total launch mass is ~82,000t. The core of the propulsion system is a set of a thousand laser stations, each 30km wide, in sun-synchronous orbit around Mercury. These together convert solar energy into laser beams with a combined total power of 1300TW. The beams are united by a ‘beam combiner’ at Mercury’s L-2 point, and ultimately passed through a 300-km transmitter lens between Saturn and Uranus (!).Forward’s ingenious deceleration method is to split the craft’s sail in two; a torus, detached from a central section, goes ahead of the craft, collects a fresh 2-year burst of laser light from the solar system, and reflects it back on the residual 300km-diameter sail still attached to the ship, thus slowing it down.
Forward always thought big – and Rocheworld shows Forward’s imagination at its staggering best. However the design brings vulnerabilities, as Forward dramatises. The propulsion system needs continuous support on the ground for decades; in the end political opposition nearly kills the project.
Encounter with Tiber, a 1996 novel by Apollo 11 astronaut Buzz Aldrin with sf veteran John Barnes , is about visits to Earth by ‘Tiberans’, natives of Alpha Centauri, fleeing disastrous impact events in that system. Two generations of starfaring technology are used by the Tiberans: solar sails, and later ‘zero-point energy lasers’ (of which more later). Gregory Matloff, who contributed to the theoretical development of interstellar solar sailing ( p52), was a consultant on the book, and the result shows in the detail depicted.
The starship Wahkopem Zomos is a stubby cylinder ~20m high with a habitat ring, rotated for artificial gravity. (The distances are my estimates from the text; the ship is described in alien units!) At launch it is connected to a booster stack ~100m tall; an antimatter reaction creates a plasma rocket to drive the ship into its sun, Alpha Centauri A, where its solar sail ~1000km wide is unfurled for a perihelion manoeuvre. After a second perihelion push at Alpha B, the ship is driven further by lasers. Ultimately the ship reaches ~0.4c. Deceleration is achieved by a ‘brakeloop’ of superconducting filament ~100km across; this ionises the interstellar medium, thus converting the ship’s kinetic energy to heat. The ship was to be brought back from Sol using a laser beam fired from Alpha Centauri and reflected from a separately launched 5000km mirror sail. In the event political opposition causes this system to be abandoned, stranding the crew until a more advanced ship comes to fetch them.
Lightsails are beautiful and the ride comes for free – but rockets are more fun! In my own novel Space  my hero Reid Malenfant uses a Daedalus-type pulsed-fusion rocket to make a one-way dash to an interstellar teleport node.
The ultimate rocket fuel is of course antimatter. An antimatter rocket launches the AXIS probe (Automated eXplorer of Interstellar Space) to Alpha Centauri in Greg Bear’s Queen of Angels . The journey takes fifteen years; for the first four years the probe is accelerated by the products of an antimatter-matter reaction, and then nanotechnology is used to manufacture huge ‘superconducting wings’ which decelerate the probe through drag from the Galaxy’s magnetic field. In an afterword Bear refers to Forward’s work on antimatter drives, and work by Matloff and Fennelly on the deceleration technique ( p95). Bear gives only one precise figure about his ingenious ship, and that’s the cost: one hundred billion dollars.
Another antimatter rocket features in one of the more detailed starship designs in recent sf: the Venture Star of James Cameron’s 2009 movie Avatar. Cameron and his consultants did a good deal of research and design work on the ship, as on many aspects of the movie, not all of which shows up on screen; however background material has been published in various tie-in media .
The Venture Star is one of a fleet of interstellar ferries, each designed to carry 200 passengers to Pandora, most of them in ‘cryosleep’ – and including ‘avatars’, the human-native hybrid bodies used to explore the moon. On its return it carries 350t of cargo, much of it the precious superconducting mineral ‘unobtainium’, which is essential for antimatter containment. The ship is over a kilometre long, much of which is strut to separate the habitable compartments from the ‘hybrid deuterium fusion / matter-antimatter engine’.
The ship is launched from the solar system with a laser push on a ‘photon sail’ 16km in diameter. During the launch a ‘mirror shield’ protects the crew compartments from the laser; during the cruise phase at 0.7c this is deployed as a shield against the interstellar medium. Deceleration is achieved using the antimatter engine. This flight sequence is reversed for the return trip.
The trouble with rockets is that you need to carry fuel. When it was first proposed in 1960 the beauty of the Bussard ramjet ( p109ff) was that it appeared to offer ultra-fast interstellar travel without the need to carry any fuel at all.
Ramjets featured in the sf of Larry Niven almost from Bussard’s first publication. The mighty Ringworld used ramjets as attitude stabilisers . Excitement reached fever pitch when Carl Sagan, for example, examined an arbitrarily long ramjet journey at an acceleration of 1g; he found that thanks to time dilation the ship could reach the boundary of the visible universe within the lifetime of a crew member .
Unfortunately, further studies into the ramjet design indicated that such marvellous dreams are probably impractical. Modified ramjet designs appeared, but these lacked the power, and indeed the romance, of the original notion. In Footfall, written by Niven with Pournelle , the fithp, invaders from Alpha Centauri, travel in what appears to be a modified ramjet of the ‘ram augmented interstellar rocket’ (RAIR) kind. The Message Bearer (chapter 16) is a mile-long cylinder, surrounded by a metallic band that appears to be the core of the ramjet system. The ship was launched with a vast spherical fuel tank attached to the nose, evidently fuel for fusion reactors which use the interstellar medium as reaction matter. The ship is turned halfway to Sol to use the ramscoop for deceleration. The mother ship incidentally carries warships which appear to use Daedalus-type pulsed-fusion drives; these are also used to manoeuvre the mother ship.
It takes several fithp generations for Message Bearer to cross from the nearest star. So much for ramjets! But the disillusion with the idea did not come early enough to quash the inspiration for what many readers regard as the quintessential starflight novel.
Poul Anderson’s Tau Zero  – the novel after which of course the Tau Zero Foundation, partners in Icarus, was named – was surely inspired by Sagan’s visionary calculations. The starship Leonora Christine with its 50 crew sets off on an exploratory mission to Beta Virginis 32ly away. The ship, like ‘a dagger pointed at the stars’ (Chapter 2), comes with a gym and swimming pool! Under an ion drive it spirals out from Earth orbit at 0.1g. When it has achieved sufficient velocity the Bussard unit opens up at the rear of the craft: ‘the scoopfield webs … glistened in the sunlight, silver across starry black’ (Chapter 3). These webs are the root of a ‘field of magnetohydrodynamic forces’ thousands of kilometres across. The fields also serve as shields from the interstellar medium. Crucially, there are separate acceleration vs. deceleration ramjet systems.
A few light years out, calamity strikes. The ship hits a fast-moving nebula – and, already moving at a high percentage of lightspeed (‘low tau’), the crew find their deceleration system is wrecked. To go outside the ship to make repairs they would need to turn the acceleration system off – but that would take down the shields too, killing the crew. The ship must go on ever faster, time dilation becoming ever stronger, until the distances and times travelled become cosmological: ‘We will not be able to stop before the death of the universe … I propose we go on to the next cycle of the cosmos’ (chapter 20). Much of the book is dated now – the Bussard physics, the cosmology. But the sheer sense of wonder never dates; and many researchers into interstellar flight cite Tau Zero as a specific inspiration for their careers.
What about more exotic drives?
The exploitation of vacuum energy as a star drive seems to have been pioneered by Charles Sheffield in his story ‘All the Colours of the Vacuum’ (1981) . This is fun hard sf written by a physicist, but the drive’s details are a bit vague: ‘The available [vacuum] energies made up a quasi-continuous “spectrum” … Tuned resonators in the .. drive units selected certain wavelengths which were excited by the corresponding components in the vacuum self-energy. These “colours,” as McAndrew thought of them, could feed vacuum energy to the drive system …’
The vacuum energy (aka ‘zero point energy’) drive in Encounter with Tiber by Aldrin and Barnes , based on work by Matloff who consulted on the novel ( p132), works by manipulating the Casimir effect – the one way we do know to extract the energy. Little specific detail is given, but you get a sense of the fragility of a drive generated by manipulating plates held a few atomic diameters apart.
Vacuum energy was most famously exploited by Clarke in his 1986 novel The Songs of Distant Earth . Fleeing the impending nova explosion of the sun, the Magellan leaves for a star more than a hundred light years away. Essentially a cylinder four kilometres long, and carrying a small conscious crew supervising a million frozen sleepers, Magellan is driven at 0.2c by a ‘quantum ramjet’. In his acknowledgements Clarke cites papers by Froning in JBIS and elsewhere ( p131) for speculation on the use of vacuum energies for propulsion, but he is even vaguer than Sheffield in the details of how the drive works. In the engine room we glimpse only ‘the shrouded metal and crystal shapes, the curiously-formed flying buttresses springing from the walls of the chamber, the pulsing constellations of lights, the sphere of utter blackness that, even though it was completely featureless, somehow seemed to be spinning … “One day it will make us masters of the Galaxy”’ (chapter 53).
However, with assistance from Alan Bond of the BIS, Clarke went into interesting detail on one often-neglected aspect of star missions: the ablation shielding required to protect such a craft from the interstellar medium. To travel ~50ly at 0.2c the Magellan requires ~100,000t of water ice moulded into a conical shield. In this book the shielding actually drives the story; to replenish the ice the Magellan is forced to call at an Earthlike planet called Thalassa 50ly from Earth, previously colonised by a ‘seedship’; the plot develops from the interaction of the Thalassans with the starship crew.
An Armada of Dreams
For fun I have attached a table summarising the characteristics of some of these paper starships, with Daedalus as a comparison .
One impression that stands out from these careful dramatisations is how appallingly difficult interstellar travel is. Schemes based on current or near-future technologies are huge, elaborate, rickety, fragile – the thousands of laser stations, the sails and lenses the size of moons. We even have to combine technologies; we put lightsails on antimatter rockets. There is surely only so far you can take this. Once the ancient Greeks put sails on rowing boats, but the result was not a nuclear submarine. On the other hand, Columbus’s ships were inadequate to meet the challenges of an Atlantic crossing – but he went anyway.
And in the pages of a novel you glimpse the sheer mine-bending scale of the engineering needed. Forward’s mighty laser beam was visible from Barnard’s Star , and Clarke’s quantum-ramjet Magellan is still visible as a 3rd magnitude star some 15 light years out ( chapter 55). How wonderful to imagine human engineering visible across interstellar distances.
|Parameter||Daedalus ||‘Crazy Eddie Probe’ ||Prometheus ||Wahkopem Zomos ||Venture Star ||Message Bearer ||Leonora Christine ||Magellan |
|Destination||Barnard’s Star||‘New Caledonia System’ (35ly from Mote)||Barnard’s Star||Sol (from Alpha Cen)||Alpha Cen||Sol (from Alpha Cen)||Beta Vir (32ly)||‘Sagan 2’ (100ly)|
|Duration||48y||40y||~6y (subj)||Generations||~5y (subj)||500y|
|Dimensions||54,000t||450,000t; sail ~1000s km||Payload 3500t, total at launch 82000t||20m-long payload||Length 1.6km, payload 350t||1 mile long||4km long|
|Crew||1 Motie plus miniatures||20||~200 (most sleepers)||50||1 million (sleepers)|
|Primary propulsion||Fusion pellets||Lightsail, laser and solar||Lightsail, 1300TW laser||1000km solar sail, EM braking||16km lightsail, antimatter drive||RAIR||Bussard ramjet||Quantum ramjet|
|Secondary propulsion||Plasma rocket||Pulsed fusion||Ion drive|
|Shielding||64m dia., 9mm thick beryllium||Mirror shield||Bussard field||100,000t of water ice|
 R. Heinlein, Universe, Astounding Science Fiction, May 1941.
 G. Bear, Hull Zero Three, Orbit, 2010.
 S. Baxter, Ark, Gollancz, 2009.
 G.L. Matloff, Deep Space Probes, Springer, 2010.
 P. Gilster, Centauri Dreams, Springer, 2004.
 E.E. Smith, Skylark of Space, Amazing Stories, 1928.
 C. Smith, ‘The Lady Who Sailed the Soul’, Galaxy, 1960.
 A.C. Clarke, ‘The Wind from the Sun’, Boy’s Life Magazine, March 1964.
 L. Niven and J. Pournelle, The Mote in God’s Eye, Simon & Schuster, 1974.
 R. Forward, Rocheworld, Baen Books, 1990 (first publication 1982, much expanded edition 1990).
 B. Aldrin, J. Barnes, Encounter with Tiber, Warner, 1996.
 S. Baxter, Space, HarperCollins, 2000.
 G. Bear, Queen of Angels, Warner Books, 1990.
 M. Wilhelm and D. Mathison, James Cameron’s Avatar: An Activist Survival Guide, HarperCollins, 2009.
 L. Niven, Ringworld, Ballantine, 1970.
 I. Shklovskii and C. Sagan, Intelligent Life in the Universe, Holden-Day, Inc, 1966.
 L. Niven and J. Pournelle, Footfall, Del Rey, 1985.
 P. Anderson, Tau Zero, Doubleday, 1970.
 C. Sheffield, ‘All the Colours of the Vacuum’, Analog, 1981.
 A.C. Clarke, The Songs of Distant Earth, Del Rey, 1986.
 A. Bond et al, Project Daedalus Final Report, British Interplanetary Society, 1978.
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